Traditionally, vacuum cleaner bags have been fabricated from a relatively porous cellulosic, i.e., paper, substrate. Vacuuming efficiency is good with such paper vacuum bags, that is, the soil is removed from the surface being vacuumed. However, vacuuming efficiency, according to this definition, is more a function of the vacuum force generated by the vacuum cleaner than a measure of vacuum bag performance.
The paper substrates are sufficiently porous to permit an air flow through the clean bag of about 25 to 50 cubic feet per minute (cfm) per square foot of substrate and are adequate to retain particulate matter of above 10 microns. This accounts for most of the weight of the soil to be vacuumed. However, because the paper vacuum bag is porous, the smaller particles initially pass through the paper vacuum bag medium. As a result, the smaller particles, that is, "dust," is exhausted into the air from the vacuum itself. This can be observed by viewing the exhaust of the vacuum backlighted by sunlight. Indeed, it is not uncommon for there to be dust covering furniture in a room previously dusted prior to vacuuming.
During use, the pores of the paper vacuum bag become plugged with particles of dirt. As one might expect, the plugging of the pores of the paper vacuum bag assists in capture of the smaller particles. However, this occurs only after several uses of the vacuum, and often when the bag has been filled to a significant degree. Moreover, at least until the paper vacuum bag is quite plugged, the inherent porosity of this filter medium permits the particles entrapped in its pores to be dislodged and replaced by similarly sized particles, a phenomenon known as seepage penetration The effect, then, is the same--the smaller particles are exhausted into the atmosphere.
The reentry of small particles of less than about 10-20 microns into the vacuumed room is, of course, irksome because the room has not been cleaned meticulously. However, the particles of less than about 20 microns include pollen (about 20 microns), skin scale (about 15 microns), spores (0.25 to 3 microns), fungi (about 2 microns), bacteria (0.25 to 2 microns) and fair amounts of dust (5-100 microns). These air contaminants cause serious allergies or occasion the transmittal of various diseases, e.g., flu. Accordingly, the removal or reduction of such finely sized contaminants from the vacuumed surface without releasing them through the vacuum cleaner exhaust is particularly desirable. Indeed, these particles are better left on the surface being vacuumed than releasing them into the atmosphere.
Attempts have been made to provide vacuum cleaner bags which are better in retaining the smaller particles within the bag, and not exhausting them into the atmosphere.
Thus, U.S. Pat. No. 4,589,894 to Gin discloses a vacuum cleaner bag of three ply construction comprising (a) a first outer support layer of highly porous fabric formed of synthetic fibers, the fabric having an air permeability of at least 100 m.sup.3 /min/m.sup.2 ; (b) an intermediate filter layer formed of a web comprising randomly interentangled synthetic polymeric microfibers that are less than 10 microns in diameter, has a weight of 40 to 200 g/m.sup.2, and an air permeability of about 3 to 60 m.sup.3 /min/m.sup.2, and (c) a second outer support layer disposed on the opposite side of the web having an air permeability of at least 50 m.sup.3 /min/m.sup.2. The web of the Gin vacuum cleaner bag may be made by melt-blown or solution-blown processes. Illustratively, the Examples 1-7 in Gin describe use of melt-blown polypropylene as the web ply and nylon or spun-bonded polypropylene as the support plys.
Another multiply filter medium useful for vacuum cleaner bags is disclosed in U.S. 4,917,942 to Winters. The laminate structure of Winters comprises a porous layer of self-supporting nonwoven fabric having an air permeability of 300 m.sup.3 /min/m.sup.2 and a layer of randomly intertangled nonwoven mat of electret-containing microfibers of synthetic polymer coextensively deposited on and adhered to the self-supporting nonwoven fabric. The self-support layer is, preferably, a spun-bonded thermoplastic polymer. The electret-containing mat is preferably based on a melt-blown polyolefin.
The melt-blown polyolefin fiber webs used by Gin and Winters as the filter medium are disadvantageous in that they have little structural strength. Thus, they are characterized by poor tensile and tear strengths, and cannot be fabricated into a usable vacuum cleaner bag independent of the supporting scrims. This adds to the cost of the vacuum cleaner bag, which is, of course, undesirable. Moreover, these fibers do not lend themselves to vacuum cleaner bag fabrication utilizing the type of equipment used commonly in the manufacture of vacuum cleaner bags.
It has been found that a vacuum cleaner bag characterized by excellent retention of small particles of 10 microns or less can be fabricated from a sheet of flashspun polyolefin fibers. This flashspun sheet, described in greater detail below with respect to its manufacture and properties, has excellent strength. Accordingly, vacuum cleaner bags of the present invention can be fabricated from a sheet of this material, and without the requirement for a supporting scrim. Moreover, this material, which comprises ultra-short fibers of micro diameter, can be fabricated into a nonwoven substrate with a process analogous to the manufacture of cellulosic substrates, which account for the majority of vacuum cleaner bags currently sold. Advantageously, these flashspun sheets have a uniform effective pore size distribution which permits their utilization as a vacuum cleaner bag without substantial decay in air permeability throughout its normal use--i.e., until the vacuum cleaner bag of the present invention has been essentially filled.